A radio frequency (RF) receiving device, of which a cellular radiotelephone mobile subscriber unit is a convenient example, provides an appropriate setting for describing the need for the present invention. Subscriber units are designed to operate with all cellular systems (for example: A or B) that provide competing services in a particular geographical area. Each system is assigned a number of channels with a particular channel spacing and each channel has a particular center frequency as called out in Electronic Industries Association (EIA)-553, .sctn. 2.1.1.1 "Mobile Station-Land Station Compatibility Specification", September 1989. The subscriber unit gains access to the system in which it is registered, System A for example, by scanning the access channels to determine the strongest two access channels per EIA-553, .sctn. 2.6.3.2. Upon gaining access, the subscriber unit may communicate through the System A provider with the landline system. However, the subscriber unit also receives signals from the System B provider which may produce one or more undesired signals in the subscriber unit on the same channel as the desired signal from System A. The undesired signal created from System B signals is of no use to the subscriber unit desiring service from System A. A receiver typically determines which channel to use by measuring the channels signal strength. But, a receiver cannot distinguish between the undesired and desired signals using only signal strength information. Since the undesired signal strength information may be generated by the System B provider, the system access signal may be on the undesired signal channel and the subscriber unit may roam to the undesired signal channel. If both of the two strongest signals from the access channel scan are undesired signals without access system modulation (typically 10 KHz manchester) the subscriber unit will lose service and the no service light will turn on. Thus, economical receiver technology has not reached the point where the mobile subscriber unit can operate with impunity close to one system's transmitting base station while communicating with the other.
A particular undesired signal response produced by receivers is commonly referred to as intermodulation distortion (IM). IM distortion develops in the presence of two unmodulated interfering signals so separated from the assigned input signal frequency and from each other that the n'th order mixing of the two undesired signals can occur in nonlinear electronic devices of the receiver producing a third signal whose frequency is equal to that of the assigned input signal frequency. The transfer functions of electronic devices commonly used in amplifying and mixing circuits within receivers are seldom if ever ideal--linear in the case of an amplifier, square law in the case of a mixer. The non-ideal characteristics inherent in these devices lead to IM distortion.
It is well known that a 1 dB change in signal strength of the interfering signals results in a 3 dB change in signal strength of the undesired third order IM distortion. Those wishing to understand the background for this 3:1 relationship may reference a paper presented by Richard C. Sagers of Motorola Inc. entitled "Intercept Point and Undesired Responses", presented before the 32nd IEEE Vehicular Technology Conference May 23-25, 1982. Knowing this 3:1 relationship is very helpful in detecting IM distortion in receivers and is commonly used in designing receivers to maximize the rejection of IM distortion.
Consumer radio receivers, such as an AM/FM radio, process a received voice signal through an attenuator automatically via control circuitry or when the user manually depresses the "local/DX" button in response to receiving a degraded channel. In areas where the receiver encounters a desired voice signal and a strong interfering voice signal causing third order IM distortion, the attenuator reduces the desired voice signal by the magnitude of the attenuator whereas the third order IM distortion in the receiver is reduced by three times the magnitude of the attenuator. Thus, signal reception of the desired signal is improved.
For many situations, of which a cellular radiotelephone mobile subscriber unit is merely an example, the prior art has not produced an apparatus or method of channel characterization to meet the difficult requirement of detecting IM distortion while providing for the advantage of an economical receiver.